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In astrophysics, the gravastar (a portmanteau of "gravitational vacuum star") is an object hypothesized in a 2006 paper by Pawel O. Mazur and Emil Mottola as an alternative to the black hole theory. It has the usual black hole metric outside of the horizon, but de Sitter metric inside. On the horizon there is a thin shell of matter. This solution to the Einstein equations is stable and has no singularities. Further theoretical considerations of gravastars include the notion of a nestar (a second gravastar nested within the first one).

Structure

In the original formulation by Mazur and Mottola, a gravastar is composed of three regions, differentiated by the relationship between pressure p and energy density ρ. The central region consists of false vacuum or "dark energy", and in this region p = −ρ . Surrounding it is a thin shell of perfect fluid where p = ρ . On the exterior is true vacuum, where p = ρ = 0 .

The dark-energy-like behavior of the inner region prevents collapse to a singularity, and the presence of the thin shell prevents the formation of an event horizon, avoiding the infinite blue shift. The inner region has thermodynamically no entropy and may be thought of as a gravitational Bose–Einstein condensate. Severe red-shifting of photons as they climb out of the gravity well would make the fluid shell also seem very cold, almost absolute zero.

In addition to the original thin-shell formulation, gravastars with continuous pressure have been proposed. These objects must contain anisotropic stress.

Externally, a gravastar appears similar to a black hole: it is visible by the high-energy radiation it emits while consuming matter, and by the Hawking radiation it creates. Astronomers search the sky for X-rays emitted by infalling matter to detect black holes. A gravastar would produce an identical signature. It is also possible, if the thin shell is transparent to radiation, that gravastars may be distinguished from ordinary black holes by different gravitational lensing properties, as null geodesics may pass through.

Mazur and Mottola suggest that the violent creation of a gravastar might be an explanation for the origin of our universe and many other universes because all the matter from a collapsing star would implode "through" the central hole and explode into a new dimension and expand forever, which would be consistent with the current theories regarding the Big Bang. This "new dimension" exerts an outward pressure on the Bose-Einstein condensate layer and prevents it from collapsing further.

Gravastars also could provide a mechanism for describing how dark energy accelerates the expansion of the universe. One possible hypothesis uses Hawking radiation as a means to exchange energy between the "parent" universe and the "child" universe, and so cause the rate of expansion to accelerate, but this area is under much speculation.

Gravastar formation may provide an alternative explanation for sudden and intense gamma-ray bursts throughout space.

LIGO's observations of gravitational waves from colliding objects have been found either to not be consistent with the gravastar concept, or to be indistinguishable from ordinary black holes.

Comparison with black holes

By taking quantum physics into account, the gravastar hypothesis attempts to resolve contradictions caused by conventional black hole theories.

Event horizons

In a gravastar, the event horizon is not present. The layer of positive-pressure fluid would lie just outside the "event horizon", being prevented from complete collapse by the inner false vacuum. Due to the absence of an event horizon, the time coordinate of the exterior vacuum geometry is everywhere valid.

Dynamic stability of gravastars

In 2007, theoretical work indicated that under certain conditions, gravastars as well as other alternative black hole models are not stable when they rotate. Theoretical work has also shown that certain rotating gravastars are stable assuming certain angular velocities, shell thicknesses, and compactnesses. It is also possible that some gravastars which are mathematically unstable may be physically stable over cosmological timescales. Theoretical support for the feasibility of gravastars does not exclude the existence of black holes as shown in other theoretical studies.

See also

• Acoustic metric
• Acoustic Hawking radiation from sonic black holes
• Black star (semiclassical gravity)
• Dark-energy star

References

1. ^ "Los Alamos researcher says 'black holes' aren't holes at all". Los Alamos National Laboratory. Archived from the original on 13 December 2006. Retrieved 10 April 2014.
2. McRae, Mike (20 February 2024). "Bubble-Like 'Stars Within Stars' Could Explain Black Hole Weirdness". ScienceAlert. Archived from the original on 20 February 2024. Retrieved 20 February 2024.
3. Jampolski, Daniel; Rezzolla, Luciano (15 February 2024). "Nested solutions of gravitational condensate stars". Classical and Quantum Gravity. 41 (6): 065014. arXiv:2310.13946. Bibcode:2024CQGra..41f5014J. doi:10.1088/1361-6382/ad2317. S2CID 264426808.
4. Mazur & Mottola 2024, p. 88
5. Cattoen, Celine; Faber, Tristan; Visser, Matt (21 October 2005). "Gravastars must have anisotropic pressures". Classical and Quantum Gravity. 22 (20): 4189–4202. arXiv:gr-qc/0505137. Bibcode:2005CQGra..22.4189C. doi:10.1088/0264-9381/22/20/002. ISSN 0264-9381. S2CID 10023130.
6. Sakai, Nobuyuki; Saida, Hiromi; Tamaki, Takashi (17 November 2014). "Gravastar shadows". Physical Review D. 90 (10): 104013. arXiv:1408.6929. Bibcode:2014PhRvD..90j4013S. doi:10.1103/PhysRevD.90.104013. ISSN 1550-7998. S2CID 119102542.
7. Chown, Marcus (7 June 2006). "Is space-time a superfluid?". New Scientist. Archived from the original on 2016-04-12. Retrieved 2017-11-04. It's the big bang," says Mazur. "Effectively, we are inside a gravastar. "alternative URL". bibliotecapleyades.net.
8. Chirenti, Cecilia; Rezzolla, Luciano (11 October 2016). "Did GW150914 produce a rotating gravastar?". Physical Review D. 94 (8): 084016. arXiv:1602.08759. Bibcode:2016PhRvD..94h4016C. doi:10.1103/PhysRevD.94.084016. ISSN 2470-0010. S2CID 16097346. We conclude it is not possible to model the measured ringdown of GW150914 as due to a rotating gravastar.
9. "Did LIGO detect black holes or gravastars?". ScienceDaily. October 19, 2016. Retrieved 2017-11-04.
10. "LIGO's black hole detection survives the gravastar test". Extreme Tech. 2016-10-26. Retrieved 2017-11-04.
11. "Was gravitational wave signal from a gravastar, not black holes?". New Scientist. 2016-05-04. Retrieved 2017-11-04. Our signal is consistent with both the formation of a black hole and a horizonless object – we just can't tell.
12. Cardoso, Vitor; Franzin, Edgardo; Pani, Paolo (27 April 2016). "Is the Gravitational-Wave Ringdown a Probe of the Event Horizon?". Physical Review Letters. 116 (17): 171101. arXiv:1602.07309. Bibcode:2016PhRvL.116q1101C. doi:10.1103/PhysRevLett.116.171101. ISSN 0031-9007. PMID 27176511. S2CID 206273829.
13. Stenger, Richard (22 January 2002). "Is black hole theory full of hot air?". CNN.com. Retrieved 10 April 2014.
14. Cardoso, Vitor; Pani, Paolo; Cadoni, Mariano; Cavaglià, Marco (26 June 2008). "Ergoregion instability of ultracompact astrophysical objects". Physical Review D. 77 (12): 124044. arXiv:0709.0532. Bibcode:2008PhRvD..77l4044C. doi:10.1103/PhysRevD.77.124044. ISSN 1550-7998. S2CID 119119838.
15. Chirenti, Cecilia B. M. H.; Rezzolla, Luciano (8 October 2008). "Ergoregion instability in rotating gravastars" (PDF). Physical Review D. 78 (8): 084011. arXiv:0808.4080. Bibcode:2008PhRvD..78h4011C. doi:10.1103/PhysRevD.78.084011. ISSN 1550-7998. S2CID 34564980. Archived from the original (PDF) on 4 March 2016. Retrieved 10 April 2014.
16. Rocha, P; Miguelote, A Y; Chan, R; da Silva, M F; Santos, N O; Wang, Anzhong (23 June 2008). "Bounded excursion stable gravastars and black holes". Journal of Cosmology and Astroparticle Physics. 2008 (6): 25. arXiv:0803.4200. Bibcode:2008JCAP...06..025R. doi:10.1088/1475-7516/2008/06/025. ISSN 1475-7516. S2CID 118669175.

Further reading

• Camenzind, Max (2007). Compact Objects in Astrophysics: White Dwarfs, Neutron Stars and Black Holes. Astronomy and astrophysics library. Berlin ; New York: Springer. pp. 442–445. ISBN 978-3-540-49912-1.
• George Chapline (2005-03-28). "Black holes 'do not exist'". Nature News.
• Mazur, Pawel O.; Mottola, Emil (7 February 2024). "Gravitational Condensate Stars: An Alternative to Black Holes". Universe. 9 (2): 88. arXiv:gr-qc/0109035. Bibcode:2023Univ....9...88M. doi:10.3390/universe9020088. ISSN 2218-1997. The original paper by Mazur and Mottola
• Visser, Matt; Wiltshire, David L. "Stable gravastars — an alternative to black holes?" (PDF). Retrieved 2004-10-02.
• Rezzolla, Luciano; Zanotti, Olindo (2018). Relativistic Hydrodynamics. Oxford: Oxford University Press. pp. 599–603. ISBN 978-0-19-852890-6.

External links

• Papers about gravastars on gr-qc
• Black Hole's Evil Twin - Gravastars Explained. Kurzgesagt – In a Nutshell. 3 December 2024 – via YouTube.

v t e Black holes
Outline
Types	BTZ black hole Schwarzschild Rotating Charged Virtual Kugelblitz Supermassive Primordial Direct collapse Rogue Malament–Hogarth spacetime
Size	Micro Extremal Electron Stellar Microquasar Intermediate-mass Supermassive Active galactic nucleus Quasar LQG Blazar OVV
Formation	Stellar evolution Gravitational collapse Neutron star Related links Tolman–Oppenheimer–Volkoff limit White dwarf Related links Supernova Micronova Hypernova Related links Gamma-ray burst Binary black hole Quark star Supermassive star Quasi-star Supermassive dark star X-ray binary
Properties	Astrophysical jet Gravitational singularity Ring singularity Theorems Event horizon Photon sphere Innermost stable circular orbit Ergosphere Penrose process Blandford–Znajek process Accretion disk Hawking radiation Gravitational lens Microlens Bondi accretion M–sigma relation Quasi-periodic oscillation Thermodynamics Bekenstein bound Bousso's holographic bound Immirzi parameter Schwarzschild radius Spaghettification
Issues	Black hole complementarity Information paradox Cosmic censorship ER = EPR Final parsec problem Firewall (physics) Holographic principle No-hair theorem
Metrics	Schwarzschild (Derivation) Kerr Reissner–Nordström Kerr–Newman Hayward
Alternatives	Nonsingular black hole models Black star Dark star Dark-energy star Gravastar Magnetospheric eternally collapsing object Planck star Q star Fuzzball Geon
Analogs	Optical black hole Sonic black hole
Lists	Black holes Most massive Nearest Quasars Microquasars
Related	Outline of black holes Black Hole Initiative Black hole starship Black holes in fiction Big Bang Big Bounce Compact star Exotic star Quark star Preon star Gravitational waves Gamma-ray burst progenitors Gravity well Hypercompact stellar system Membrane paradigm Naked singularity Population III star Supermassive star Quasi-star Supermassive dark star Rossi X-ray Timing Explorer Superluminal motion Timeline of black hole physics White hole Wormhole Tidal disruption event Planet Nine
Notable	Cygnus X-1 XTE J1650-500 XTE J1118+480 A0620-00 Sagittarius A* Centaurus A Phoenix Cluster PKS 1302-102 OJ 287 SDSS J0849+1114 TON 618 MS 0735.6+7421 NeVe 1 Hercules A 3C 273 Q0906+6930 Markarian 501 ULAS J1342+0928 PSO J030947.49+271757.31 AT2018hyz Swift J1644+57
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